1. Field of the Invention
The present invention relates to audio signal processors. Illustrative embodiments of the invention relate to audio mixing consoles, especially digital audio mixing consoles.
2. Description of the Prior Art
Consider the illustrative arrangement shown in FIG. 1. A stereo musical recording of an orchestra playing classical music is made using a `stereo pair` 1 of two identical directional microphones placed as close together as possible in a good listening position.
It is often necessary to increase the loudness of some component of the performance such as a voice or a quiet instrument. In FIG. 1 a flute F in the orchestra is shown as an example. The established method of doing this is to position a spot microphone 2 as close as possible to the flute and add the output of the microphone to the left and right channels of the stereo signal in some controllable proportions, using a stereo mixing console 3.
That method creates several problems because the path lengths through the air from the flute to the spot microphone 2 and to the stereo pair 1 are different. The flute signal derived from the spot microphone applied to the left and right stereo channels has a different timing to the same signal derived from the stereo pair.
This, in all cases, creates a filtering effect because at some frequencies the signals add, and others they subtract, due to the phase differences created by the differing air-path lengths. That creates an unwanted comb-filtering effect.
In addition, in general, the flute signal arrives at spot microphone 2 earlier than the flute signal at the stereo pair 1. The ear responds to the first signal to reach it (not the loudest signal) to fix the position of a stereo audio image. Thus the flute signal derived from the spot microphone can create an incorrectly positioned stereo image. To manually adjust the amplitude and delay of the spot signal is a skilled, difficult, task.